1. Field of the Invention
This invention relates to tank-type furnaces for producing flat glass. More particularly, this invention relates to a tank-type furnace for producing a flat glass made from a highly volatile, high temperature melting glass composition, such as a crystallizable glass composition.
2. Brief Description of the Prior Art
Crystallizable glasses are a special class of glass which can be heat treated to transform the glass into a semi-crystalline ceramic. The ceramic differs considerably from the original glass in physical, chemical, mechanical and electrical properties. Such ceramics are transparent or opaque and generally have a much lower thermal expansion coefficient than the original uncrystallized glass. These properties make the semi-crystalline product particularly attractive for stove top applications.
U.S. Pat. Nos. 2,920,971 to Stookey and 3,625,718 to Petticrew describe typical crystallizable glass compositions.
Crystallizable glasses which are of particular importance are those which can be transformed into the crystalline phase, beta-spodumene solid solution. Such glasses are made with the alkali metal oxide Li.sub.2 O and contain very little of the good fluxing agents Na.sub.2 O and K.sub.2 O, since these latter ingredients adversely affect the expansion coefficient of the resultant crystallized glass product. As a result, the crystallizable glasses have very high melting temperatures, that is, some 200.degree. to 400.degree.F. above that required for melting soda-lime-silica glasses.
To provide some fluxing activity, certain preferred crystallizable glass compositions contain heavy metal oxides such as ZnO. ZnO is a particularly attractive ingredient because it not only acts as a fluxing agent but also acts as a promoter for crystallization acting to increase the rate of crystallization. Also, the ZnO does not adversely affect the expansion coefficient.
Unfortunately, heavy metal oxides such as ZnO are quite volatile at the high temperatures employed in melting crystallizable glass. The high melting temperatures in conjunction with high volatility negates the use of conventional flat glass furnaces which are generally about 150 to 200 feet in overall length. The amount of energy required to keep the glass in the melt form for this long a period of travel would be prohibitive. Also, the loss of volatile constituents over this length of travel would be excessive resulting in a glass deficient in volatile constituents and of poor quality. Thus, shorter furnaces with heat applied across the entire length of the furnace are usually employed.
Producing, in a short furnace, glass which has a high melting temperature and which contains highly volatile ingredients presents quality control problems. As the glass is heated and melted, convection currents are set up in the molten melt; the flow being from the hotter to the cooler regions. Therefore, on the surface of the melt, there is a flow of glass outwardly to the side walls of the tank which act as a heat sink. At the front end of the furnace where the side walls are conventionally at a 90.degree. angle to the front end wall, surface convection currents are particularly strong because of the large side wall-end wall surface area exposure. As a result, a significant portion of the glass throughput stream will be diverted into the corner regions of the furnace rather than through a delivery canal which extends through the front end of the furnace. With conventional soda-lime-silica glasses, the diversion of the glass into the corner regions is not particularly critical, because the glass melts at a fairly low temperature and the glass-making ingredients do not contain any exceptionally volatile constituents. However, with a high temperature melting, highly volatile glass such as a crystallizable glass containing ZnO, diversion of the glass into the corner regions is undesirable. While in these corners, the glass which is at a fairly high temperature, loses its volatile constituents. The glass in the corners becomes less dense and resists any tendency to flow back into the throughput stream (which is more dense). The glass stagnates in the corners becoming more and more deficient in volatile constituents, more and more silica-rich (silica is the least volatile constituent in the glass) and becoming lighter and lighter in density. Eventually, however, as more glass is pulled into the corner areas, some of the silica-rich, volatile component-deficient glass necks out into the throughput stream at the throat of the canal. Consequently, the throughput glass in the canal and the resultantly formed glass ribbon will not be of uniform composition. The upper portions of the edges of the ribbon are of a different composition from the main body of the glass ribbon, being silica-rich and deficient in volatile components. The result is a glass of very poor quality which is optically distorted. The difference in composition of the corners from the main body of the glass can be observed by examining a cross-section of the glass under cross-polaroids. When the glass composition is a crystallizable glass, the above-described problem of ribbon inhomogeneity is particularly acute. When the glass is heat treated to crystallize it, the different compositions across the ribbon result in different rates of crystallization and warpage and cracking of the sheet.
From the above, it is apparent that a new method and furnace design were needed for making high quality flat glass from high temperature melting glass compositions containing highly volatile ingredients.